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Author
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Topic: Negative Horsepower Result on Dyno - Fact or Fiction
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Stinkynutz
Journeyman Posts: 97
From: Fabulous Las Vegas, Nevada, usa
Registered: Nov 2003
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posted 10-03-2004 11:13 AM
   
When I bought my 351W for my '65 289 swap, the seller also gave me the real wheel dyno runs (from a Dynojet roll-on) he had while the motor was in a '83 Bronco with big tires coupled to a C6. The motor has a great selection of performance parts but had a max power of only 200 HP at the wheels, and a negative horsepower of 120 at 77 mph! The tech data from Dynojet indicates that the combined numbers represent flywheel horsepower, so the total = 320 - fair enough. But does 120 hp look like a reasonable amount of power lost through the drivetrain. I using this motor in Mustang with a C4, so I'm curious about what my numbers will look like. Thanks.
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SteveLaRiviere
Administrator Posts: 43152
From: Saco, Maine
Registered: May 99
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posted 10-03-2004 06:32 PM
You mean 'net horsepower' right, not 'negative horsepower?'------------------
'70 Mustang Mach 1 351C 4V/FMX/3.25 Open
'70 Mustang Convertible 250 I6/3 speed/2.79 Open
'72 Mustang Sprint Hardtop 351C 4V/FMX/4.30 Trac Loc
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip
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Fastymz
Moderator Posts: 18703
From: Reno Nv USA MEM#1240
Registered: Apr 2001
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posted 10-03-2004 06:39 PM
That sound about right for a 4x4 with a C6 and large tires. I'd say your set up should give up about 20-30% to the drivetrain.------------------
SCOOP oddly obsessed with big scoops on little Mustangs 65 coupe,351w,C4,Big Boss 429 hood scoop,8" 3.40 TracLoc. My Pics
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Stinkynutz
Journeyman Posts: 97
From: Fabulous Las Vegas, Nevada, usa
Registered: Nov 2003
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posted 10-03-2004 07:32 PM
No Steve, I meant negative horsepower. According to the Dynojet web site (maker of the Winpep dyno software), when you do a roll-on (RO) dyno run, you can also perform a negative horsepower run. You mash the gas and at a desired speed, push in the clutch (freewheel), and measure the resistance. The printout produces a negative horsepower curve which, according to the software tech data, is supposed to give you an accurate reading of entire drivetrain hp loss throughout various mph's. I just thought -120 hp sounded like a heck of a lot, but I've heard rumors about what power bandits C6's are. Apparently, this dyno run confirms that.
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SteveLaRiviere
Administrator Posts: 43152
From: Saco, Maine
Registered: May 99
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posted 10-03-2004 08:04 PM
I learned something new. Thanks.  ------------------
'70 Mustang Mach 1 351C 4V/FMX/3.25 Open
'70 Mustang Convertible 250 I6/3 speed/2.79 Open
'72 Mustang Sprint Hardtop 351C 4V/FMX/4.30 Trac Loc
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip
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Daniel Jones
Gearhead Posts: 813
From: St. Louis, MO
Registered: Aug 99
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posted 10-03-2004 10:28 PM
I don't I believe your driveline is eating that kind of power.
120 HP is a LOT of of heat. Think about the cooling system that
is required to cool an engine. A transmission cooler is tiny by
comparison. Some power is lost in the driveshaft, rear end, and
even the tires but I still find 120 HP hard to believe. Also,
remember it takes very little power to run a vehicle at say 60 MPH,
maybe only 25 HP. Dan Jones
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N266fords
Gearhead Posts: 1404
From: Spokane ,WA USA
Registered: Apr 2003
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posted 10-03-2004 10:58 PM
dont forget that was on a big tired c-6 4x4 that means transfer case rearend frontend two drive shafts ect.
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Stinkynutz
Journeyman Posts: 97
From: Fabulous Las Vegas, Nevada, usa
Registered: Nov 2003
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posted 10-03-2004 11:02 PM
Dan, check out page 58 of this manual. Am I misunderstanding something? I do remember from my old thermo classes (I'm an engineer) that the total heat from a 120 hp loss is a bunch.http://www.dynojet.com/downloads/manuals/98118101.05_WP6User_Auto.pdf
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Toronado3800
Gearhead Posts: 851
From: St. Louis, MO
Registered: Jul 2001
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posted 10-04-2004 03:29 AM
I'd believe it especially on a 4x4. The rear wheel dyno numbers in carcraft are always lots below the engine on a stand with only an electric water pump attached numbers.Heck, I suppose when you have -200 HP on a 200 HP motor your not going far.
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indyphil
Gearhead Posts: 2398
From: Senoia, G.A. USA
Registered: Jul 2002
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posted 10-04-2004 12:30 PM
Sounds like an awful lot.I mean for a regular car (say mine)
C4 tranny, 2.79 open rear end, stock converter 200hp at the flywheel (ish)
Im probably making 160 at the wheels (ish)
So im eating 40hp (20%) between the engine and the ground (ish) truck has 350 (ish)
The numbers suggest hes losing 34% which I suppose is feasible with a C6 and 4 wheel drive plus big tires. With a plain C4 I would suggest using 20-25% for your current application. ------------------
'68 coupe, '66 289 C code
engine, edel 600cfm carb, performer intake, dual exhaust http://www.geocities.com/ottouk_77/68mustang.htm
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